Dental compositions comprising filler and resin matrix are used as dental materials for repairing a portion missing from a tooth or for forming dental prostheses including artificial teeth. The dental compositions are provided in the form of paste. The dental composition changes from paste to hardened product by curing the resin matrix comprised in the dental compositions. Dental compositions when in the form of paste are desired to have an appropriate filling property that allows the paste being easily filled into a cavity of tooth, and a good preservation stability. The cured products formed from the dental compositions are desired to have a good mechanical strength, smooth surface, and adequate color tone comparable with natural teeth. In addition, cured dental products preferably provide cutting and grinding sensation comparable with that of natural teeth.
Dental compositions tend to shrink in association with polymerizing and curing the resin matrix. The degree of volumetric shrinkage is called a rate of polymerization shrinkage. When a dental composition with a high rate of polymerization shrinkage is filled to a missing part of a tooth and cured there, strain and gap may occur at the interface between the tooth and the dental composition, resulting in a poor adhesiveness and edge sealing. When such dental compositions as above are used as a filling material to fill a large cavity of tooth, there is a tendency for secondary caries to occur therein. Conventional dental compositions are insufficient in mechanical strength, e.g., bending strength and abrasion resistance when they are in the form of cured product.
When the dental compositions with a high rate of polymerization shrinkage are cured outside the oral cavity to from a dental block or a dental prosthesis, they have a strong propensity to develop high internal stress and cracks on the inside, resulting in the poor mechanical strength. When dental blocks are cut and ground to form a dental prosthesis, it is difficult to form an accurate dental prosthesis because the shape of the delicate or thin parts of the prosthesis tend to change.
Conventional dental materials had comprised relatively large size quartz particles as filler. Dental materials containing such large size filler possessed practically acceptable degree of shrinkage of the material upon polymerization of the material in the form of paste. The mechanical strength was also practically acceptable. However, the cured materials could provide only unsatisfying grinding and polishing feelings.
Dental materials containing ultrafine silica particles having particle size from 0.01 to 0.05 μm were developed. The dental material can provide a grinding sensation similar to that of natural teeth and can easily provide a smooth surface. However, the amount of the ultrafine particle filler in the dental material is limited due to high viscosity of the material containing a high amount of the filler when it is in the form of paste. Therefore, a cured product obtained from the paste of the dental material had unfavorable mechanical strength and abrasion resistance. In addition, degree of shrinkage of the material upon polymerization was also unfavorable. Hybrid-type dental materials having advantages of both aforementioned dental materials had been developed (JP-A 63-88110).